1. Field of the Invention
The invention relates to ground fault circuit interrupters and, more particularly, to ground fault circuit interrupters designed to provide protection to human beings accidentally causing a ground fault.
2. Description of the Prior Art
Circuits for supplying alternating current to electrical apparatus normally include two or more conductors. It is desirable that all current flowing through the electrical apparatus being powered travel through these conductors; that is, all currents going toward the load on one conductor of the circuit should return by one of the other conductors, rather than through the ground. In practice this is impossible to achieve for there is always at least a small amount of "leakage current". This leakage current results from less than perfect insulation surrounding the supply circuit conductors and the electrical apparatus being powered. The leakage current, rather than returning through the conductors, returns through the earth or ground. In 120 volts AC circuits, leakage currents should not be more than a few milliamperes. Another occasion in which current traveling out on one of the conductors will not return through one of the other conductors is a ground fault. This occurs when an object or person completes a path from one of the conductors to ground, thereby allowing current flow from the conductor to ground.
People-protecting ground fault circuit interrupters designed for use on 120 volts AC supplies are designed to interrupt the power circuit upon ground fault currents above a certain level. This is done by measuring the net current flow through all of the conductors of the circuit being protected. Under ideal conditions, as described above, the net current flow should be zero, since all of the current traveling toward the load on one conductor should return on one of the other conductors. People-protecting circuit breakers are designed to ignore a small amount of net current flow which occurs because of leakage current. However, when a ground fault occurs which produces a net current flow through the detecting means greater than about 5 milliamperes (5 ma), the people-protecting ground fault circuit interrupter will trip or open. The use of a 5 ma trip level has evolved from measurements of the "can't-let-go" current level of a large population sample which indicated that 99.5% of the population could release an object at high potential if the current flow through their bodies was less than 5 ma.
It is desirable to utilize people-protecting circuit breakers on higher voltage circuits. However, the expected leakage current on circuits supplied with these higher voltages may exceed the 5 ma level. Little use has been made of people-protecting circuit breakers on higher voltage circuits because of leakage current and measurement errors which exceed the equivalent of 5 ma ground current, especially during overload or surge current conditions. It would be desirable to provide a people-protecting ground fault circuit interrupter which could provide protection at voltage levels above 120 volts AC.